Reduction of head-to-head agglutinability of mammalian spermatozoa in the epididymis has been considered to be one of changes related to sperm maturation and to be associated with regulation of expression of sperm fertility. We previously reported that boar sialoglycoprotein (epididymal plasma anti-agglutinin) (EPAA) was one of regulators of agglutinability and that interaction of EPAA with spermatozoa was dependent on degree of posttranslational modifications in this molecule. The present objective was to characterize the primary structure, glycosylation and phosphorylation of EPAA by mass spectrometry. The results obtained by mass spectrometric analysis are shown below. 1. EPAA has molecular mass of 19379 Da (ESI-MS), which is different from that estimated by SDS-PAGE (25 kDa). Also,a similar mass spectrum was obtained for anti-agglutinin purified from seminal plasma, indicating almost no change in the structure of EPAA in this stage of maturation. 2. Mass spectra of EPAA after deglycosylation and tryptic digestion (peptide mass fingerprint) (ESI MS) are unique, suggesting EPAA is a novel protein. This suggestion has been already confirmed by N-terminal amino acid sequence analysis. 3. Comparison of mass spectra of EPAA before and after deglycosylation (MALDI-MS) reveals that EPAA is not heavily glycosylated, but glycosylation is present. 4. Research on phosphorylation is in progress now. These results (1-3) have provided valuable information on the regulatory mechanism of the reduction of sperm agglutinability.